A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to accurately apply a desired pattern onto the substrate, alignment marks are provided on the substrate, and the lithographic apparatus is provided with an alignment system. The alignment system is configured to perform measurements and determine the positions of the alignment marks provided on the substrate therefrom.
It is desirable to reduce the size of the alignment marks so that more space on the substrate is available for applying a desired pattern onto the substrate. Reducing the size of the alignment marks may reduce the reproducibility and/or the accuracy of measurements made using the alignment system. It is desirable to provide an alignment method that allows the size of the alignment marks to be reduced without affecting the reproducibility and/or the accuracy of measurements made using the alignment system. Alternatively, it may be desirable to maintain the size of the alignment marks and improve the reproducibility and/or the accuracy of measurements made using the alignment system.